This invention relates to a zoom lens assembly having an auto-focus function and more particularly to a zoom lens assembly provided with a ring for at least presetting a distance, driven by a motor and effecting an auto-focus function.
Recently, various kinds of auto-focus cameras have been developed. In the field of a single-lens reflex camera, an exchangeable lens is offered which can effect an auto-focus function. An auto-focus unit utilizing light passing through the lens (abbreviated TTL light) has been developed, wherein a light measuring section of the auto-focus unit detects a position of an image forming plane for a foreground object; and a distance-presetting ring is driven in order to eliminate an error which might arise in the determination of a distance between the image forming plane and a film surface. Development of an auto-focus unit utilizing the TTL light has made it possible to shorten the length of time required to detect the position of the image forming plane and provide a picture whose focus precision is improved.
Moreover, a zoom lens assembly with an auto-focus function has been developed. With this type of a zoom lens unit, light which is reflected from a foreground object, but which has not passed through the zoom lens is measured. In other words, it has been impossible with the zoom lens unit to adopt the method of measuring the TTL light. The reason for this drawback is that it is impossible to resolve problems arising from the effect on the depth of focus according to changes on the focal length caused by the rotation of a zoom ring. Namely, even where the image forming planes produced in telephotography and widephotography are displaced from the film surface by the same distance, a foreground object is actually more remote from the film surface in the widephotography than in the telephotography, in a case where the depth of focus changes.
For instance, when a distance between a foreground object and the film surface varies from the infinite .infin. to 1 m, then the position of the image forming plane is moved about 5.2 mm in a case where the focal length is set at 70 mm, and about 1.3 mm in a case the focal length is set at 35 mm. Where, therefore, the position of the image forming plane has been found to be displaced 1.3 mm from the film surface, then the distance-presetting ring has to be rotated in order to meet a prescribed distance of 1 m, in the case where the focal length is set at 70 mm. In the case where the focal length is set at 35 mm, the distance-presetting ring has to be rotated to meet a prescribed distance of 3.8 m. As described above, a measured distance between the film surface and the image forming plane has to be changed to coincide with the required rotation of the distance-presetting ring to define a prescribed distance, according to a focal length set by the zoom ring. For the reason given above, it has been impossible to adopt the TTL light measuring method for the zoom lens unit.